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Consider this code snippet:

Browse Source 
float t1(int argc) {
  return (argc == 1123) ? 1.234f : 2.38213f;
}

We would generate truly awful code on ARM (those with a weak stomach should look
away):

_t1:
  movw   r1, #1123
  movs   r2, #1
  movs   r3, #0
  cmp    r0, r1
  mov.w  r0, #0
  it     eq
  moveq  r0, r2
  movs   r1, #4
  cmp    r0, #0
  it     ne
  movne  r3, r1
  adr    r0, #LCPI1_0
  ldr    r0, [r0, r3]
  bx     lr

The problem was that legalization was creating a cascade of SELECT_CC nodes, for
for the comparison of "argc == 1123" which was fed into a SELECT node for the ?:
statement which was itself converted to a SELECT_CC node. This is because the
ARM back-end doesn't have custom lowering for SELECT nodes, so it used the
default "Expand".

I added a fairly simple "LowerSELECT" to the ARM back-end. It takes care of this
testcase, but can obviously be expanded to include more cases.

Now we generate this, which looks optimal to me:

_t1:
  movw   r1, #1123
  movs   r2, #0
  cmp    r0, r1
  adr    r0, #LCPI0_0
  it     eq
  moveq  r2, #4
  ldr    r0, [r0, r2]
  bx     lr
  .align  2
LCPI0_0:
  .long   1075344593  @ float 2.382130e+00
  .long   1067316150  @ float 1.234000e+00



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110799 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2010-08-11 08:43:16 +00:00
parent 7b4d31176e
commit de2b151dbf
3 changed files with 76 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

53
lib/Target/ARM/ARMISelLowering.cpp
View File

@ -483,9 +483,9 @@ ARMTargetLowering::ARMTargetLowering(TargetMachine &TM)
setOperationAction(ISD::SETCC, MVT::i32, Expand);
setOperationAction(ISD::SETCC, MVT::f32, Expand);
setOperationAction(ISD::SETCC, MVT::f64, Expand);
setOperationAction(ISD::SELECT, MVT::i32, Expand);
setOperationAction(ISD::SELECT, MVT::f32, Expand);
setOperationAction(ISD::SELECT, MVT::f64, Expand);
setOperationAction(ISD::SELECT, MVT::i32, Custom);
setOperationAction(ISD::SELECT, MVT::f32, Custom);
setOperationAction(ISD::SELECT, MVT::f64, Custom);
setOperationAction(ISD::SELECT_CC, MVT::i32, Custom);
setOperationAction(ISD::SELECT_CC, MVT::f32, Custom);
setOperationAction(ISD::SELECT_CC, MVT::f64, Custom);
@ -2314,6 +2314,52 @@ ARMTargetLowering::getVFPCmp(SDValue LHS, SDValue RHS, SelectionDAG &DAG,
return DAG.getNode(ARMISD::FMSTAT, dl, MVT::Flag, Cmp);
}
SDValue ARMTargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
SDValue Cond = Op.getOperand(0);
SDValue SelectTrue = Op.getOperand(1);
SDValue SelectFalse = Op.getOperand(2);
DebugLoc dl = Op.getDebugLoc();
// Convert:
//
// (select (cmov 1, 0, cond), t, f) -> (cmov t, f, cond)
// (select (cmov 0, 1, cond), t, f) -> (cmov f, t, cond)
//
if (Cond.getOpcode() == ARMISD::CMOV && Cond.hasOneUse()) {
const ConstantSDNode *CMOVTrue =
dyn_cast<ConstantSDNode>(Cond.getOperand(0));
const ConstantSDNode *CMOVFalse =
dyn_cast<ConstantSDNode>(Cond.getOperand(1));
if (CMOVTrue && CMOVFalse) {
unsigned CMOVTrueVal = CMOVTrue->getZExtValue();
unsigned CMOVFalseVal = CMOVFalse->getZExtValue();
SDValue True;
SDValue False;
if (CMOVTrueVal == 1 && CMOVFalseVal == 0) {
True = SelectTrue;
False = SelectFalse;
} else if (CMOVTrueVal == 0 && CMOVFalseVal == 1) {
True = SelectFalse;
False = SelectTrue;
}
if (True.getNode() && False.getNode()) {
EVT VT = Cond.getValueType();
SDValue ARMcc = Cond.getOperand(2);
SDValue CCR = Cond.getOperand(3);
SDValue Cmp = Cond.getOperand(4);
return DAG.getNode(ARMISD::CMOV, dl, VT, True, False, ARMcc, CCR, Cmp);
}
}
}
return DAG.getSelectCC(dl, Cond,
DAG.getConstant(0, Cond.getValueType()),
SelectTrue, SelectFalse, ISD::SETNE);
}
SDValue ARMTargetLowering::LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const {
EVT VT = Op.getValueType();
SDValue LHS = Op.getOperand(0);
@ -3687,6 +3733,7 @@ SDValue ARMTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
return Subtarget->isTargetDarwin() ? LowerGlobalAddressDarwin(Op, DAG) :
LowerGlobalAddressELF(Op, DAG);
case ISD::GlobalTLSAddress: return LowerGlobalTLSAddress(Op, DAG);
case ISD::SELECT: return LowerSELECT(Op, DAG);
case ISD::SELECT_CC: return LowerSELECT_CC(Op, DAG);
case ISD::BR_CC: return LowerBR_CC(Op, DAG);
case ISD::BR_JT: return LowerBR_JT(Op, DAG);

1
lib/Target/ARM/ARMISelLowering.h
View File

@ -335,6 +335,7 @@ namespace llvm {
SelectionDAG &DAG) const;
SDValue LowerGLOBAL_OFFSET_TABLE(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerBR_JT(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerSELECT(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerBR_CC(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerFCOPYSIGN(SDValue Op, SelectionDAG &DAG) const;

25
test/CodeGen/ARM/select.ll
View File

@ -1,5 +1,6 @@
; RUN: llc < %s -march=arm | FileCheck %s
; RUN: llc < %s -march=arm -mattr=+vfp2 | FileCheck %s --check-prefix=CHECK-VFP
; RUN: llc < %s -mattr=+neon,+thumb2 -mtriple=thumbv7-apple-darwin | FileCheck %s --check-prefix=CHECK-NEON
define i32 @f1(i32 %a.s) {
;CHECK: f1:
@ -65,3 +66,27 @@ define double @f7(double %a, double %b) {
%tmp1 = select i1 %tmp, double -1.000e+00, double %b
ret double %tmp1
}
; <rdar://problem/7260094>
;
; We used to generate really horrible code for this function. The main cause was
; a lack of a custom lowering routine for an ISD::SELECT. This would result in
; two "it" blocks in the code: one for the "icmp" and another to move the index
; into the constant pool based on the value of the "icmp". If we have one "it"
; block generated, odds are good that we have close to the ideal code for this:
;
; CHECK-NEON: _f8:
; CHECK-NEON: movw [[REGISTER_1:r[0-9]+]], #1123
; CHECK-NEON-NEXT: movs [[REGISTER_2:r[0-9]+]], #0
; CHECK-NEON-NEXT: cmp r0, [[REGISTER_1]]
; CHECK-NEON-NEXT: adr [[REGISTER_3:r[0-9]+]], #LCPI
; CHECK-NEON-NEXT: it eq
; CHECK-NEON-NEXT: moveq [[REGISTER_2]], #4
; CHECK-NEON-NEXT: ldr
; CHECK-NEON: bx
define arm_apcscc float @f8(i32 %a) nounwind {
%tmp = icmp eq i32 %a, 1123
%tmp1 = select i1 %tmp, float 0x3FF3BE76C0000000, float 0x40030E9A20000000
ret float %tmp1
}